J O U R N A L O F C H E M I S T R Y Materials Room-temperature synthesis of monodisperse mixed spinel (CoxMn1-x)3O4 powder by a coprecipitation method Young-Il Jang, HaifengWang and Yet-Ming Chiang* Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 24th August 1998, Accepted 25th September 1998 Spinel oxide Co3O4 has traditionally been synthesized by thermal decomposition of cobaltous salts at temperatures of 250–900 °C under oxidizing conditions.Here, we report a solution synthesis route that yields mixed spinel oxide (CoxMn1-x)3O4 at room temperature in air. (Co0.75Mn0.25)3O4 solid solutions were synthesized by coprecipitation and oxidation of Co(OH)2 and Mn(OH)2 from mixed aqueous solutions of Co- and Mn-salts.The spinel oxide phase was stabilized by crystallization of Mn3O4 resulting from the oxidation of Mn(OH)2 in the mother liquor. While undoped Mn3O4 is tetragonal, the (Co0.75Mn0.25)3O4 solid solution has a cubic symmetry due to the reduced Mn3+ concentration. The resulting powder is spherical with an average particle diameter of ca. 0.1 mm and a narrow size distribution. Transmission electron microscopy (TEM) showed that individual particles are nearly single crystalline but consist of a mosaic of multiple nanocrystallites. This observation supports an aggregation mechanism of formation for the nearly monodispersed particles.We therefore anticipated that under autoxidation conditions Introduction where Mn3O4 is favored, a predominantly Co(OH)2 material Co3O4 has extensively been investigated as an electrocatalyst doped with Mn(OH)2 would exhibit enhanced autoxidation for both O21–4 and Cl2 evolution.5 Although it has the draw- of Co(OH)2 to Co3O4 spinel.The Mn3O4 was expected to act back of lower long-term stability compared to recently devel- as a stabilizer for Co3O4 formation. The direct precipitation oped RuO2 catalysts,6 Co3O4 has the advantage of lower cost of mixed metal spinel oxides has previously been reported for while retaining adequate performance.This spinel compound ferrites,21 including those doped with Co,22 Mn,23 and Ni.24 is also receiving wide attention as a promising electrochromic material.7 There are numerous reported procedures for the preparation of Co3O4, all essentially based on the thermal Experimental decomposition of cobaltous salts at temperatures varying between 250 and 900 °C under oxidizing conditions as reviewed Co(NO3)2 6H2O (Alfa Aeasar, 99.5%), Mn(NO3)3 6H2O by Sugimoto and Matijevic.8 High temperature firing typically (Aldrich, 98%) and LiOH H2O (Alfa Aeasar, 98%) were used results in oxides with surface area of only a few square meters for preparing the starting aqueous solutions.A special aspect per gram.9 Because oxides are used in finely divided form, of our synthesis procedure is the use of LiOH as precipitating electrocatalytic properties often depend strongly on morpho- reactant rather than previously used reactants such as NH4OH. logical as well as electronic factors. Therefore, particles of The ammonium ion is known to retard the rate of autoxidation uniform shape with a large surface area and a narrow size of Mn(OH)2,20 mitigating against use of NH4OH.Preliminary distribution are generally required.10 examinations to determine appropriate autoxidation con- EVorts have previously been made to synthesize fine Co3O4 ditions for Mn3O4 were performed with a 0.1 M Mn(NO3)3 powder at low temperatures by solution methods. Sugimoto solution at room temperature in ambient air atmosphere.and Matijevic8 obtained Co3O4 particles of cubic shape with Results from these experiments were used to determine the uniform edge length (ca. 0.1 mm) by heating cobalt acetate reaction conditions for obtaining (Co0.75Mn0.25)3O4. solution at 100 °C in the presence of O2 gas.Recently, 0.1 M aqueous solutions of Co(NO3)2 and Mn(NO3)3 in a Furlanetto and Formaro10 reported that mixtures of Co3O4 Co5Mn=351 atomic ratio were added dropwise to a LiOH and CoOOH can be obtained by mixing Na3Co(NO2)6 solusolution that was vigorously stirred and monitored to keep tion with NH4NO3 solution while purging with N2 gas. Upon the pH at ca. 11. This is near the minimum solubility conditions heating at 70–100 °C, they observed that CoOOH transformed for both Co(OH)2 and Mn(OH)2.25 For comparison, the to Co3O4, resulting in spherical particles with diameter of precipitation of Co3O4 was also attempted from 0.1 M 0.2–0.3 mm.Co(NO3)2 solution under the same conditions. In each The present work describes a novel room-temperature instance, the precipitate was aged in the mother solution for coprecipitation synthesis method which yields mixed spinel 12 h at room temperature in air with continuous stirring, (CoxMn1-x)3O4.This composition is of interest for several followed by rinsing to remove Li+ and NO3- species with reasons. Like the endmembers Co3O41–5 and Mn3O4,11,12 distilled water as described in ref. 26. Finally the precipitate (CoxMn1-x)3O4 solid solutions can be applied as catalysts for was atomized into liquid nitrogen, and the frozen droplets oxygen reduction,13 and considered to be amongst the mixedwere freeze-dried (VirTis Consol 12LL, Gardiner, NY). spinel candidates for active catalysts.4,14–18 Like Co3O4, The present synthesis method has also been used to prepare Mn3O4 can easily be obtained by thermal decomposition of lithium intercalation compounds, in which case LiOH solution manganous salts.19 However, this compound can also be is added after the rinsing step and before freeze-drying.26–29 prepared by the so-called ‘autoxidation’ of Mn(OH)2 in The eVect of residual LiOH on the present materials was also alkaline aqueous solution.It is known that mixtures of MnO2, studied, as an extreme case where the precipitating reactant is Mn2O3 or Mn3O4 can be obtained by oxidation of Mn(OH)2, not completely removed.The powders were characterized by depending on many factors such as temperature, pH, nature of ions in the solution, and the rate of air or oxygen flow.20 X-ray diVraction (XRD) using a Rigaku diVractometer J. Mater. Chem., 1998, 8, 2761–2764 2761(RTP500RC) with Cu-Ka radiation and by transmission electron microscopy (TEM) using a JEOL-200CX instrument.Results and discussion The XRD pattern of the oxide precipitated from Mn(NO3)3 solution is shown in Fig. 1. Mn3O4 appears as the predominant phase after freeze-drying with no other Mn-containing minor phases being detectable. Miller indices (hkl) are indexed for the tetragonal phase (space group I41/amd) in Fig. 1. Lattice parameters calculated from the XRD data using Cohen’s least squares method30 are: a=5.764 A° , c=9.452 A° . These values agree well with the data in JCPDS (#24-734).31 The tetragonal symmetry of Mn3O4 spinel is known to result from the cooperative Jahn–Teller distortion of Mn3+ ions in octahedral sites.32 According to ref. 32, the c/a ratio decreases from 1.64 Fig. 2 Powder XRD pattern of (Co0.75Mn0.25)3O4 (with hkl in Fd39m) to 1.50 (equivalently from 1.157 to 1.054 in space group obtained by coprecipitation from Co(NO3)2 and Mn(NO3)3 solution F41/ddm) when Mn3O4 is lithiated to LiMn3O4, indicating mixed with LiOH solution (%: Li2CO3). that the degree of tetragonal symmetry decreases when lithiation reduces Mn3+ to Mn2+.Since the Mn3O4 obtained the same conditions. As shown in Fig. 3, Co(OH)2 appears as in this study has a c/a ratio of 1.64, it appears to be largely the major phase (space group P3m1), and CoOOH as a minor unlithiated after freeze-drying. This is consistent with TEM phase (space group R39m). It is thus clear that when Co(OH)2 results on LiCoO2 produced by the same method,26 where the is coprecipitated with Mn(OH)2, the spinel phase is stabilized LiOH remains as a separate amorphous phase after freeze- by Mn3O4 resulting from the autoxidation of Mn(OH)2.drying. One can notice from Fig. 1 that Li2CO3 exists as a Miller indices (hkl) are indexed in Fig. 2 for the cubic mixedminor phase. This is due to minor reaction of residual LiOH spinel phase.The lattice parameter calculated from the XRD with CO2 to form the carbonate phase during sample handling; data is a=8.168 A° , inbetween that of Co3O4 (8.084 A° )34 and LiOH is a well-known CO2 absorbent.33 However, the residual MnCo2O4 (8.226 A° ),35 as is expected from its intermediate lithium salts LiOH or Li2CO3 are easily removed from the Mn concentration.Fig. 4 shows that the variation in cubic present powder by washing as they are water soluble. spinel lattice parameter is approximately linear (Vegard’s law) Having confirmed that Mn3O4 is obtained under the present with Mn concentration, increasing upon substituting the larger precipitation and autoxidation conditions, we employed the Mn ions for Co ions.36 same experimental conditions to synthesize (Co0.75Mn0.25)3O4.Fig. 2 shows the XRD pattern of powder obtained by coprecipitation and autoxidation of Co(OH)2 and Mn(OH)2 from their mixed nitrate solution. The diVraction peaks match very well with the XRD patterns of either Co3O4 or MnCo2O4 as reported in JCPDS (#43-1003 and #23-1237).34,35 Considering the fact that MnCo2O4 in ref. 35 was synthesized by heating Co- and Mn-nitrate precursors at 760 °C, one can conclude that the present oxide obtained at room temperature is nearly as well crystallized as high temperature fired materials.The slight peak broadening in Fig. 1 and 2 is consistent with fine crystallite size, as discussed later. Note that (Co0.75Mn0.25)3O4 has a cubic symmetry (space group Fd39m), indicating that the concentration of Mn3+ is lower than the critical concentration for the cooperative Jahn–Teller distortion to a tetragonal symmetry.The formation of Co3O4 spinel was not observed in Fig. 3 Powder XRD pattern of Co(OH)2 (with hkl in P3m1) obtained precipitation experiments with Co(NO3)2 solution alone under by precipitation from Co(NO3)2 mixed with LiOH solution (%: Li2CO3; 1: CoOOH).Fig. 1 Powder XRD pattern of Mn3O4 (with hkl in I41/amd) obtained Fig. 4 Lattice parameter of (Co0.75Mn0.25)3O4 in comparison with by precipitation from Mn(NO3)3 solution mixed with LiOH solution (%: Li2CO3). Co3O434 and MnCo2O4.35 2762 J. Mater. Chem., 1998, 8, 2761–2764direct observations with higher resolution of a single particle using TEM. Fig. 6 shows a bright field TEM image of a single oxide particle and corresponding selected area diVraction pattern.We observe diVraction contrast variations within each particle showing clearly that each particle is composed of much smaller nanocrystallites. However, sharp crystalline diVraction spots are observed in the selected area diVraction pattern in Fig. 6, indicating that each particle is overall singlecrystalline. It appears that the nanocrystallites have aggregated to form a particle in which the crystallites are suYciently misoriented with respect to one another to provide diVraction contrast, yet are well aligned enough to yield sharp reflections in the selected area diVraction pattern (mosaic structure). These results strongly support the aggregation mechanism for the present system. Conclusion A cobalt-rich mixed spinel oxide (Co0.75Mn0.25)3O4 has been synthesized at room temperature by the coprecipitation of Mn Fig. 5 TEM micrograph of (Co0.75Mn0.25)3O4 particles obtained by and Co hydroxides from mixed nitrate solutions. Mn3O4 coprecipitation from Co(NO3)2 and Mn(NO3)3 solution mixed with resulting from the autoxidation of Mn(OH)2 stabilizes the LiOH solution. mixed spinel phase under conditions where the undoped Co precursor produces Co(OH)2.The resulting particles are Fig. 5 shows a TEM bright-field image of the nearly single-crystalline spheres (diameter ca. 0.1 mm), and (Co0.75Mn0.25)3O4 obtained in this study. Spherical particles have a narrow size distribution. Each particle is composed of of ca. 0.1 mm with a narrow particle size distribution are nanometer-scale crystallites, supporting an aggregation mechobserved. The uniform particle size obtained in this study is anism of particle formation.This new route is simple and noteworthy because it has generally been believed that it is cost-eVective, and is suitable for the synthesis of chemically diYcult, if not impossible, to obtain monodispersed metal homogeneous mixed cobalt spinel oxides of high surface area (hydrous) oxide colloids by the reaction of a strong base with and uniform particle size distribution for such applications as a metal salt solution.37 The much smaller particle size that we catalysis and electrochromic materials.obtained compared to that of the Co3O4 of ref. 10 (0.2–0.3 mm) is probably due to our lower synthesis temperature (25 °C vs.Acknowledgments 70–100 °C). The Li2CO3 and LiOH phases resulting from LiOH added after the rinsing step and before freeze-drying This work has been funded by the INEEL University Research are seen surrounding the spherical particles. However, without Consortium. The INEEL is managed by Lockheed Martin addition of LiOH after rinsing, those lithium-containing phases Idaho Technology Company for the U.S.Dept. of Energy, were rarely observed with TEM. Idaho Operations OYces, under contract no. DEIn order to explain the formation of precipitates with AC07–94ID13223. We used instrumentation in the Shared uniform size, the LaMer model,38,39 based on a short single Experimental Facilities at MIT, supported by NSF Grant burst of nucleation followed by uniform growth has been No. 9400334-DMR. Y.I.J. also acknowledges a fellowship previously used. However, more recently, an aggregation mech- from the Ministry of Education, Korea. anism in which each particle forms a large number of smaller subunits has been proposed by Matijevic, as reviewed in refs. References 37 and 40. In order to address this matter, we have conducted 1 C.Iwakura, A. Honji and H. Tamura, Electrochim. Acta, 1981, 26, 1319. 2 P. Rasiyah and A. C. C. Tseung, J. Electrochem. Soc., 1983, 130, 365. 3 B. E. Conway and T. C. Liu, Mater. Chem. Phys., 1989, 22, 163. 4 R.-N. Singh, M. Hamdani, J.-F. Koenig, G. Poillerat, J. L. Gautier and P. Chartier, J. Appl. Electrochem., 1990, 20, 442. 5 R. Boggio, A. Carugati, G. Lodi and S.Trasatti, J. Appl. Electrochem., 1985, 15, 335. 6 P. Siviglia, A. Daghetti and S. Trasatti, Colloids Surf., 1983, 7, 15. 7 F. Svegl, B. Orel, M. G. Hutchins and K. Kalcher, J. Electrochem. Soc., 1996, 143, 1532. 8 T. Sugimoto and E. Matijevic, J. Inorg. Nucl. Chem., 1979, 41, 165. 9 D. Pope, D. S. Walker and R. L. Moss, J. Colloid Interface Sci., 1977, 60, 216. 10 G. Furlanetto and L.Formado, J. Colloid Interface Sci., 1995, 170, 169. 11 T. Yamashita and A. Vannice, J. Catal., 1996, 163, 158. 12 A. Maltha, H. F. Kist, T. L. F. Favre, H. G. Karge, F. Asmussen, H. Onishi, Y. Iwasawa and V. Ponec, Appl. Catal. A: General, 1994, 115, 85. 13 M. Sugawara, M. Ohno and K. Matsuki, J. Mater. Chem., 1997, 7, 833. Fig. 6 Bright field TEM image and corresponding selected area 14 M.Hamdani, J. F. Koenig and P. Chartier, J. Appl. Electrochem., 1988, 18, 561. diVraction pattern of a (Co0.75Mn0.25)3O4 particle obtained by coprecipitation from Co(NO3)2 and Mn(NO3)3 solution mixed with 15 J. L. Gautier, A. Restovic and P. Chartier, J. Appl. Electrochem., 1989, 19, 28. LiOH solution. J. Mater. Chem., 1998, 8, 2761–2764 276316 D. Panayotov, M.Khristova and D. Mehandjiev, J. Catal., 1995, 29 B. Huang, Y.-I. Jang, Y.-M. Chiang and D. R. Sadoway, J. Appl. 156, 219. Electrochem., in press. 17 J. Ziolkowski, A. M. Maltha, H. Kist, E. J. Grootendorst, 30 B. D. Cullity, in Elements of X-Ray DiVraction, Addison and H. J. M. de Groot and V. Ponec, J. Catal., 1996, 160, 148. Wesley, Massachusetts, 2nd edn., 1978, p. 363. 18 J.Ghose and K. S. R. C. Murthy, J. Catal., 1996, 162, 359. 31 JCPDS-International Centre for DiVraction Data, #24-734. 19 O. Bricker, Am. Mineral., 1965, 50, 1296. 32 M. M. Thackeray, W. I. F. David, P. G. Bruce and 20 A. R. Nichols, Jr. and J. H. Walton, J. Am. Chem. Soc., 1942, J. B. Goodenough, Mater. Res. Bull., 1983, 18, 461. 64, 1866. 33 W. A. Hart and O. F. Beumel, Jr., in Comprehensive Inorganic 21 T.Sugimoto and E. Matijevic, J. Colloid Interface Sci., 1980, Chemistry, ed. A. F. Trotman-Dickenson, Pergamon, New York, 74, 227. 1973, p. 352. 22 H. Tamura and E. Matijevic, J. Colloid Interface Sci., 1982, 90, 34 JCPDS-International Centre for DiVraction Data, #43-1003. 100. 35 JCPDS-International Centre for DiVraction Data, #23-1237. 23 Z. X. Tang, C. M. Sorensen, K. J. Klabunde and 36 R. D. Shannon and C. T. Prewitt, Acta Crystallogr., Sect. B, 1969, G. C. Hadjipanayis, J. Colloid Interface Sci., 1991, 146, 38. 25, 925. 24 C.-L. Huang and E. Matijevic, Solid State Ionics, 1996, 84, 249. 37 E. Matijevic, Chem.Mater., 1993, 5, 412. 25 C. F. Baes, Jr. and R. E. Mesmer, in The Hydrolysis of Cations, 38 V. K. LaMer, Ind. Eng. Chem., 1952, 44, 1269. Krieger, Florida, 1986, pp. 219, 238. 39 V. K. LaMer and R. J. Dinegar, J. Am. Chem. Soc., 1950, 72, 26 Y.-M. Chiang, Y.-I. Jang, H. Wang, B. Huang, D. R. Sadoway 4847. and P. Ye, J. Electrochem. Soc., 1998, 145, 887. 40 E. Matijevic, Langmuir, 1994, 10, 8. 27 G. Ceder, Y.-M. Chiang, D. R. Sadoway, M. K. Aydinol, Y.-I. Jang and B. Huang, Nature, 1998, 392, 694. 28 Y.-I. Jang, B. Huang, Y.-M. Chiang and D. R. Sadoway, Electrochem. Solid-State Lett., 1998, 1, 13. Paper 8/06653A 2764 J. Mater. Chem., 1998, 8, 2761–2764